The present invention relates to a target material for magneto-optical recording medium having a low magnetic permeability which is suitable particularly for magnetron sputtering, and a process for producing the target material.
Magneto-optical recording media have come to be used as rewritable recording media with a large capacity in storage devices of computers or for filing of imagerial informations. They have come to be used also as small-sized recording media called "minidisc" for music or data recording.
In these magneto-optical recording media, the magneto-optical characteristics of an amorphous thin film composed mainly of a rare earth element and an element of the iron group are usually utilized.
The thin film used for magneto-optical recording media is usually prepared by sputtering deposition.
There are various methods for the sputtering, for example, a method of sputtering simple substances of a rare earth element and an element of the iron group, respectively, at the same time as separate targets (co-sputtering method); and a method of sputtering a target previously obtained by alloying or compositing an element which is desired to be formed into a thin film.
Of such targets, the composite type targets for magneto-optical recording medium disclosed in Japanese Patent Laid-Open Nos. 62-70550 and 1-25977, i.e., the targets comprising a rare-earth element containing phase composed mainly of a rare earth element and/or an intermetallic compound or alloy of a rare earth element and an element of the iron group, and a metallic phase composed mainly of an element of the iron group, have come to be widely used because they are advantageous in that the component distribution in a film can be made uniform by microstructure control of the target, as compared with the compound type target disclosed in Japanese Patent Laid-Open No. 1-149959.
Since a rare earth element contained in a target is very active, one or more additive elements are often added for improving the corrosion resistance of a thin film. For example, Japanese Patent Laid-Open No. 1-247571 reports a method for improving the corrosion resistance of a thin film by using a target material containing 15% by atom or less of at least one additive element selected from the group consisting of Ti, Al, Cu, Cr, Nb, Ta, Pd and Pt.
Of the above-mentioned sputtering methods, magnetron sputtering suitable for increasing deposition rate has recently become a leading method. This magnetron sputtering comprises generating a leakage magnetic flux on the surface of a target material to confine a plasma near the surface of the target material.
However, the above-mentioned composite type target disclosed in Japanese Patent Laid-Open No. 1-25977 contains ferromagnetic phases such as Fe and Co, resulting in high saturation magnetic flux density (high Bs) of a target material and hence a high magnetic permeability thereof. Therefore, an insufficient leakage is generated from the surface of the target material, resulting in problems such as a lowered film-forming efficiency and a lowered use efficiency of the target material due to a progress of formation of local convexities.
Accordingly, there is eagerly desired a target material for magneto-optical recording medium having a low magnetic permeability.
As a target material for magneto-optical recording medium which can have a reduced magnetic permeability and is suitable for magnetron sputtering, there has recently been reported in Japanese Patent Laid-Open No. 4-365860 a target having a structure comprising a composite phase composed of a crystallized metal of the iron group finely and uniformly dispersed in a matrix made of an intermetallic compound of a rare earth metal and a metal of the iron group (hereinafter referred to merely as "intermetallic compound"), a rare-earth metal phase, and an intermetallic compound phase composed of a reaction phase of the aforesaid composite phase with the aforesaid rare earth metal phase.
The target disclosed in Japanese Patent Laid-Open No. 4-365860 has a reduced magnetic permeability by virtue of the composite phase composed of a crystallized metal of the iron group finely dispersed in an intermetallic compound.
The target disclosed in Japanese Patent Laid-Open No. 4-365860 is effective in reducing the magnetic permeability but has the following defect. Since the target has the rare-earth metal phase as a pure rare-earth metal phase and the intermetallic compound phase formed by heat treatment, its structure is coarse. Therefore, the difference in angle of sputtered atoms between the rare earth metal and the intermetallic compound in sputtering affects the component distribution in a film formed, so that it is difficult to make the component distribution in the film uniform.
Said target is disadvantageous also in that because of the coarse structure, a pre-sputtering time, i.e., a time required for the composition of the film to attain equilibrium in early sputtering, is long.
The magnetic characteristics of an amorphous magnetic thin film with perpendicular magnetization composed mainly of a rare earth element and a metal of the iron group which is used as a magneto-optical recording film, are very sensitive to the composition of the film. In particular, only a slight change of the content of the rare earth element in the film causes drastic changes of coercive force and Kerr rotation angle. Therefore, in order to assure stable magnetic characteristics of a magneto-optical recording film, a target material capable of giving a uniform rare-earth element distribution in the film in a wide film formation area is desired when a magnetic layer is formed by sputtering.